Old Phone Cables Open Seabed to Science

Published: August 24, 1999

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In the next step, the team sent Jason down. Using a joystick and a television monitor aboard the Thompson, the craft's pilot, Will Sellers, slowly steered Jason into position, clamped the jaws of its maneuvering arm on the 1 1/4-inch cable, and cut it.

Next came the hardest part: snagging the severed cable with an 800-pound grapnel and hauling the cut end up to the ship. Mr. Sellers had to make sure that the grapnel grabbed the cable at least 16,400 feet away from the cut to create a counterbalance as the cable was hauled up.

''It was like positioning a slippery strand of cut spaghetti on the tine of a fork, making sure that there was enough weight of spaghetti on the loose end to keep the strand from sliding off in the other direction,'' Dr. Chave said.

The ship's crew then had to haul up six miles of cable weighing nearly 24,000 pounds -- the maximum weight the ship was capable of handling. The risky operation took an entire day.

Once the crew had wrestled the cable aboard the ship they powered electricity into it and used it to make a telephone call to the National Science Foundation in Washington. The cable worked perfectly, even though it had lain unused for nine years on the ocean floor.

But time and again unexpected problems arose. When the crew began lowering the cable and a ''termination frame'' that served as a connection between the cable and an outlet, a chain broke and both cable and frame fell to the ocean bottom. Fortunately the frame fell in a favorable position, so that when the junction box was lowered, Jason's arm was able to complete the setup by connecting the frame with the junction box. Finally, a seismometer built by the University of Hawaii was lowered into position nearby and plugged into the junction box.

Almost immediately, seismic signals began flowing to Hawaii, joining the global torrent of signals from more than 100 other sensors contributing to the IRIS network. After the Woods Hole team returns the repaired seismometer to the sea floor next month, the scientists hope the cable will continue to work for up to 30 years.

Anyone can use the data produced by the network, Dr. Chave said. Other groups, including one in Japan, are also exploiting old telephone cables, although IRIS network is the most extensive.

''The more evenly you can collect seismic signals from sensors all over the world,'' he said, ''the better you can tomographically image the structure deep inside the earth. It's a little like taking a clinical CAT scan using an inward-looking telescope. For one thing, the seismic data can tell you about the differential rotation of liquid metal in the earth's core -- a key factor causing variations in the earth's magnetic field.''

In collaboration with a scientist at Bell Laboratories Dr. Chave is also using 10 abandoned cables, all with one end reaching land somewhere, as passive sensors to measure deep ocean currents and changes in the ionosphere. As a current of sea water flows through the earth's magnetic field, an electric current is generated in the water, and a resulting voltage shows up in a cable with one end grounded in sea water. Current is also induced in an ocean cable by the flow of electricity through the ionosphere -- a layer of the atmosphere 50 miles above the earth's surface.

Three years ago the Navy announced that it was abandoning some of the hydrophone sensors it had used to track potentially hostile ships and submarines. Some of these sensors and their associated cables and electronics have been made available for civilian research.

''All this activity is really expanding the reach of geophysical science,'' Dr. Chave said. ''We have to thank technological obsolescence for giving us some wonderful tools.''

Chart: ''Reusing a Cut Cable'' A severed telephone cable some three miles below the ocean's surface is being transformed into a thousand- mile-long extension cord connecting a power station in Makaha, Hawaii, to an underwater observatory. The shore-based power station pumps 5,000 volts of electricity through the cable to a termination frame. From there, electricity is transferred to a junction box, which acts as an outlet for a seismometer and other devices. The cable also carries the seismometer signal back to scientists on the surface. (Source: Woods Hole Oceanographic Institution)